The hydroformylation reaction is of great commercial importance for the preparation of very large quantities of derivatives generally known by the term “oxo alcohols”. In the hydroformylation of propylene, the ratio of linear n-butyraldehyde to co-product isobutyraldehyde (N/Iso ratio) products has great importance in the efficient use of propylene for preparation of n-butanol, 2-ethylhexanol and derivatives of these from the linear n-butyraldehyde product. There is a need to attain high N/Iso ratio product in many business cases. Likewise, if the hydroformylation of higher molecular weight linear alpha-olefins is carried out, there is a need to selectively prepare the linear isomer aldehyde product as opposed to the less valuable branched isomer.
The use of triarylphosphine modifier, or ligand, as a co-catalyst with rhodium was disclosed in U.S. Pat. Nos. 3,527,809 and 4,247,486. The preferred ligand was triphenylphosphine. The patent art indicated that butyraldehyde N/Iso ratios of 8/1 to 12/1 were achievable when large quantities of ligand were used, typically up to 10% by weight in the catalyst solution. Triphenylphosphine is known as a monodentate ligand, meaning that the ligand molecule has one phosphorus atom to coordinate with the rhodium catalyst. Further studies indicated that the rhodium catalyst is more selective for preparing high N/Iso ratios when a high concentration of triphenylphosphine is present, allowing two ligands to coordinate with the rhodium during the catalyst cycle. The use of high concentrations of monodentate ligand and reduced reactor temperatures were also discovered to allow the catalyst to be more stable for long operating times as disclosed in U.S. Pat. No. 4,277,627.
The design and use of bi-dentate ligands with rhodium has been particularly successful in developing new hydroformylation catalyst systems capable of preparing aldehyde products with high N/Iso ratios. The use of sterically hindered bis-phosphites of substituted 2,2′-biphenol in combination with rhodium prepare butyraldehyde with N/Iso ratios of 30/1 U.S. Pat. No. 4,885,401. Van Leeuwen et. al. (Organometallics 2002, 21, pp. 3873-3883) disclosed the use of a ligand, bis(dipyrrolylphosphoramidite) ester of 2,2′-biphenol, with rhodium that is highly selective for the preparation of linear aldehyde products from 1-hexene. These two bidentate ligands are characterized by having the phosphorus atom bound in sigma bonds to oxygen and or nitrogen heteroatoms. While being selective for making linear products, these classes of phosphorus compounds are prone to decomposition with time by acid catalyzed reactions with aldehyde and alcohol coproducts present in the reactor. In the case of the commercialized bis-phosphite catalyst system, many methods have been disclosed in the patent literature to mitigate against the natural course of this class of ligand to react with the products of the hydroformylation reaction as disclosed in U.S. Pat. Nos. 5,929,289 and 5,364,950.
Other bidentate ligands based on phosphorus being bound to three carbon atom linkages have been successful in the preparation of aldehyde product with differing degrees of N/Iso ratio selectivity. 1,1′-bis(diphenylphosphino)ferrocene ligand and substituted versions have been reported to prepare heptanal with 10/1 N/Iso ratios with rhodium as disclosed in U.S. Pat. Nos. 4,193,943 and 5,789,624. The bidentate ligand “DIOP” in combination with rhodium produced butyraldehyde in a N/iso ratio of about 4/1, U.S. Pat. No. 4,201,714. 1,2-bis(diphenylphosphinomethyl)benzene in combination with rhodium produced butyraldehyde in a 2.28/1 N/Iso ratio, in U.S. Pat. No. 4,960,949. A bidentate ligand, alpha, beta-bis(diphenylphosphino)-2-ethyltoluene in combination with rhodium produced butyraldehyde with a 5.9/1 N/Iso ratio, in U.S. Pat. No. 4,774,362.
Bidentate ligands based on 2,2′-bis(diphenylphosphinomethyl)-1,1′-biphenyl and derivatives thereof have been useful as catalyst ligands in combination with rhodium to produce active hydroformylation catalysts that produce butryraldehyde with N/Iso ratios in excess of 25/1. These are disclosed in U.S. Pat. Nos. 4,694,109, 4,755,624 and 4,760,194. Another bidentate ligand based on a derivative of 2,2′-bis(dibenzophospholylmethyl)-1,1′-biphenyl produced butyraldehyde N/Iso ratios in excess of 100/1, U.S. Pat. No. 5,332,846.
The diphosphorus bidentate ligands, generally known as triorganophosphine bidentates are based on phosphorus atoms being bound to three carbon atoms. They have an advantage of catalyst stability against chemical decomposition reactions brought about by reaction with aldehyde product and alcohol coproduct present in the hydroformylation reactor. In spite of this advantage, triorganophosphine ligand catalyst systems can undergo “intrinsic deactivation” during rhodium catalyzed hydroformylation, especially in the case when using triphenylphosphine ligand. This was disclosed in some detail in U.S. Pat. No. 4,277,627. The patent indicated that high ligand/Rh mole ratios and relatively low reaction temperatures stabilized the active catalyst during hydroformylation against intrinsic deactivation. Intrinsic deactivation manifests itself by the gradual darkening of recovered catalyst solution due to the formation of inactive compounds that incorporate multiple rhodium atoms bound by decomposition products of degraded triorganophosphine ligands. These dark materials are generally known as rhodium clusters. Thus a stable catalyst is observed to remain a typically light yellow color, while ligands less stable undergo a progressive darkening, through amber to orange to eventually dark amber and brown as these inactive materials accumulate in the catalyst.
X. Zhang disclosed the use of a “tetraphosphine” ligand, namely 2,2′,6,6′-tetrakis(diphenylphosphinomethyl)-1,1′-biphenyl “Advances in Synthetic Catalysis” 2007, 349, pp. 1582-1586 that they assert is capable of producing nonanal or heptanal with a high linear/branched product ratios using 1-octene or 1-hexene respectively in combination with rhodium under hydroformylation conditions. They further demonstrate that having the tetradentate ligand of their definition permits the catalyst to produce linear/branched ratio product in higher ratios than the corresponding 2,2′-bis(diphenylphosphinomethyl)-1,1′-biphenyl ligand, BISBI, the catalyst of U.S. Pat. No. 4,694,109, at higher reaction temperatures. They assert in their article that having extra phosphorus bonding sites available on the ligand molecule helped to stabilize the catalyst to attain higher N/Iso ratio selectivity at higher temperature. While this constitutes an improvement in the art of hydroformylation, their ligand of record is relatively synthetically inaccessible if large quantities are required as would be the case in a commercial hydroformylation application.
The tetraphosphine of Zhang is prepared by a sequence involving 1) ozonation of pyrene at −78 degrees Celsius 2) careful reduction of the resulting ozonide by sodium iodide at −78 degrees Celsius 3) isolation and purification of the resulting 1,1′-biphenyl-2,2′,6,6′-tetracarboxaldehyde 4) reduction of this by sodium borohydride to a tetra-alcohol derivative 5) conversion of the tetra-alcohol to a tetrabromide derivative with phosphorus tribromide 6) conversion of the tetrabromide to a tetrachloride using lithium chloride 7) reaction of the tetrachloride with lithium diphenylphosphide to prepare the final “tetraphosphine”. The preparation to the tetrabromide was disclosed by Zhang in the reference of I. Agranet et. al. J. Org. Chem. 44, pp. 1936-1941 (1979)
Despite the improvements in the art, there remains a need to have an active hydroformylation catalyst that is stable for long periods of time that produces products with high N/Iso ratios and is economically accessible for commercial application.